Originally, this post was supposed to be a simple one, just about my thoughts as I was listening to HP last night. As I was writing, however, it became an in-depth analysis of cause and effect!
It is a sad fact that in order for me to fall asleep at night, I need to be listening to one of my books on tape. And for me "books on tape" means listening to my Harry Potter CDs. I am currently in the middle of Book 4 (my favorite): Harry Potter and the Goblet of Fire. And it occurred to me last night that Harry makes a small mistake with huge consequences! Here's the situation...

Harry is on his way back to the Gryfindor dormitory late one night after spending a few hours in the prefect's bathroom to figure out his egg clue for the second task. He checks his Marauder's Map to make sure the coast is clear, but sees something fishy: Bartemious Crouch is in Professor Snape's office! Harry wonders what he is doing there because he is supposedly too unwell to work or come to any of the tournament events! His curiosity overtakes him, and he decides to make his way to Snape's office to check it out. Unfortunately, he doesn't pay attention to what he is doing and he puts his foot onto the trick step and is caught. He drops his egg (and his map!), which causes a commotion and brings Snape and Filch to the scene. Harry, while under his invisibility cloak, tries to reach the map with his wand to wipe it clean, but can't reach it from where he stands. Eventually "Moody" joins as well, and he can see Harry due to his magical eye. "Moody" sees the map on the step and points to it, mistakenly believing it to be Snape's, at which point Harry is able to sign to "Moody" to not let Snape pick it up. "Moody" uses a summoning charm and he gets a hold of the map instead. Harry does not get it back.

Do you see Harry's mistake? I even added the emphasis to help! If Harry had only used a summoning charm, instead of just trying to wipe the map clean, the whole course of the books would have changed! We know that he can perform a summoning charm, as he had just used one very successfully in the first task. He wouldn't have had to worry about anyone hearing him whisper the spell yet, as Filch had not yet arrived, and the egg was still wailing away anyway!

If Harry had used a summoning charm, grabbed the map, and been able to see it under his cloak while Filch and Snape were talking, he would have seen that the "Moody" who arrives at the scene, was not Moody at all! It was actually Bartemious Crouch. He would have been puzzled, sure, but he would not have known at this point that it was actually Barty Crouch, Jr. He could have pointed it out on the spot to Snape and Filch, but he probably would have waited to discuss it with Ron and Hermione, or even Dumbledore. In which case, the imposter Moody would have been caught long before he was ever able to send Harry to Voldemort. Cedric would never have died, and Voldemort could never have used Harry's blood to come back!

It all sounds very good, but perhaps it wasn't a mistake after all. If Voldemort could not have used Harry's blood to come back, he would have found another "enemy's" blood to use (as Wormtail had suggested). Harry would never have been able to tell Dumbledore in all certainty that Voldemort was back, and Dumbledore would not have restarted the Order of the Pheonix. Additionally, Voldemort would not have Harry's blood running through his veins, so in the very end, when Harry gives up his life, he would have been gone for good!

So at first, I really did believe that Harry made a fatal error in not using a summoning charm to retrieve his Marauder's Map. But after a closer analysis, we can see that perhaps it was all for the best! It saved Harry's life, leading to him being able to take down Voldemort once and for all.

All from one moment when Harry didn't use a spell.

I am sure if we examined it further, we could continue to trace out more and more effects from Harry's one action (or inaction). But at this point, I think I'm done with it all!